1. Field of the Invention
The present invention relates to a ceramic slurry composition for use in manufacturing ceramic green sheets and to a preferable method for manufacturing the composition. More particularly, the present invention relates to improvement of a binder for use in manufacturing the slurry from a ceramic raw material powder.
2. Description of the Related Art
In complying with the requirements for miniaturization, weight reduction and implementation of higher densities of electronic parts, the variety and production amount of multilayer ceramic electronic parts such as multilayer ceramic capacitors are growing more and more, wherein the parts are produced by methods comprising the steps of: forming internal conductor films such as electrodes on ceramic green sheets; stacking such ceramic green sheets followed by subjecting them to compression bonding so as to obtain a raw laminate; and sintering ceramic components contained in the ceramic green sheets as well as electroconductive components contained in the internal conductor films at the same time by baking the raw laminate.
Ceramic green sheets for use in manufacturing such multilayer ceramic electronic parts are usually required to be made thinner. On the other hand, there are other occasions when the ceramic green sheets are required to be made thicker. At any rate, it is important that the ceramic green sheets have a thickness with little variation and do not have pores, etc., and that the ceramic raw material powder contained in the sheets has excellent dispersion characteristics. Regarding this point, a wet type sheet forming method for forming the ceramic green sheets is preferable to a dry type press molding method using a granulated ceramic raw material powder.
In the wet type sheet forming method, a ceramic slurry containing a ceramic raw material powder is prepared. For this purpose, conventionally, polyvinyl butyral or the like is used as a binder, and an organic solvent such as an alcohol or an aromatic solvent is used as a solvent.
However, use of an organic solvent creates great restrictions in the handling environment. Accordingly, a water-soluble binder, which is soluble in an aqueous solvent such as water itself, has been proposed recently. Among such water-soluble binders, a water-soluble acrylic binder that contains a relatively large amount of a hydrophobic component, tends to be easily adsorbed onto a ceramic raw material powder comprising a hydrophobic component, and therefore, is able to provide an ideal dispersion system with excellent dispersion characteristics. Furthermore, it has the advantage that the ceramic green sheets obtained from a slurry with the binder are less hygroscopic and thus show a smaller level of degradation due to environmental humidity. Furthermore, the same levels of sheet strength and elongation rate are obtained as those of ceramic green sheets using an organic binder such as polyvinyl butyral.
However, a water-soluble acrylic binder containing a conventional hydrophobic component generally has a high solution viscosity. A slurry using the binder also has a high viscosity, in general. Accordingly, the slurry has lower flow as well as degraded dispersion characteristics for a ceramic raw material powder, tending to make it difficult to obtain uniform ceramic green sheets.
A method for decreasing the viscosity of the slurry by increasing the amount of the aqueous solvent to be added or by decreasing the solution viscosity by decreasing the molecular weight of the binder has been proposed to solve these problems.
However, various problems will be encountered when the above-described methods are employed. For example, when ceramic green sheets having a thickness of not less than 60 xcexcm are formed while increasing the amount of an aqueous solvent, the drying characteristics will be degraded, resulting in the generation of cracks on the obtained multilayer ceramic electronic parts. When a binder with a smaller molecular weight is employed, the mechanical properties of the ceramic green sheets such as tensile strength or elongation rate will be decreased.
Accordingly, it is an object of the present invention to provide a ceramic slurry composition with which the above-described problems can be solved, and also to provide a preferable method for manufacturing the composition.
The present invention is first directed to a ceramic slurry composition comprising a ceramic raw material powder, a water-soluble acrylic binder and an aqueous solvent, and is characterized in that the water-soluble acrylic binder has a weight average molecular weight of from about 10,000 to 500,000, and an inertial square radius of not more than 100 nm in the aqueous solvent in order to solve the above-described technical problems.
When such a ceramic slurry composition is used, it is possible to provide a ceramic slurry with a low viscosity by decreasing the solution viscosity of the binder alone and without decreasing the molecular weight of the water-soluble acrylic binder containing a hydrophobic component. Furthermore, the slurry has good dispersion characteristics for a ceramic raw material powder and good flow characteristics as well as excellent characteristics for forming ceramic green sheets. Thus, high-density ceramic green sheets can be obtained which have excellent drying characteristics.
Another aspect of the present invention is characterized in that the water-soluble acrylic binder contained in the ceramic slurry composition according to the present invention has a number of aggregated molecules as denoted by X and a second virial coefficient as denoted by Y in the range satisfying the following relationship: Yxe2x89xa6xe2x88x920.0002X2xe2x88x920.0004X+0.0051 (where Xxe2x89xa71 and Yxe2x89xa70.00023).
The present invention is also directed to a preferable method for manufacturing the ceramic slurry composition. The method for manufacturing the ceramic slurry composition is characterized in that it comprises the steps of:
obtaining a binder precursor by dispersing at least acrylic acid and an acrylate into a solvent; obtaining a water-soluble acrylic binder by subjecting the binder precursor to high pressure dispersion under a pressure of not less than about 100 kg/cm2 and not more than about 1,750 kg/cm2; and mixing the water-soluble acrylic binder, a ceramic raw material powder, and an aqueous solvent.
Other objects, advantages, and novel features of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawing and tables.